I want to point out that I'm not aware of ANY of these add ons being produced by Nintendo themselves. Anytime you have one company making something for another's product the quality can be questionable. Of course this only pertains to the battery pack. I imagine any company can make a quality cover or mat.
Also as far as the cover goes--I think if you have kids using it a lot then it might be nice, but remember none of these things are required for you to enjoy the system and can always be picked up later if need arises.
This is one of the reasons people think video games are so expensive. They buy a whole bunch of unecessary accessories that drive up the start up price. (A little pet peeve of mine

)